Frequency control



C. FICHANDLER FREQUENCY CONTROL Filed Oct. 5, 193 1 May 7, 1935.

Patented May 7, 1935 This invention relates broadly to methods of varying alternating current circuit impedances and to the applications of said. methods fo'r'frei quency modulation and control.

5 Heretofore, frequency modulation has been ac complished usually by varyingthe capacity ofja condenser in a resonant oscillating circuit in ac- I cordance with the signal impulse. This system of modulation is not satisfactory'because the low 10 solutely necessaryrto locate the voice input dis tantly of the radio transmitter, and for'this rea son it is customary toconvert the voice signals into electrical impulses'by means of a microphone, and after conducting said electrical impulses to the transmitter, to reconvert them'into cillating condenser through the medium of electro-mechanical means, such as a loud-speaker. Remote control for radio telegraphy has been accomplished by the use of a relay, the armature of 25 which activates a,va'riable condenser.

in radio-telephony are undesirable, more particularly because .the stiffness and inertia of said devices cause distortion; and such devices are 30 impractical for signals of a very high frequency,

such as televisionlsignals for example. 1 J

An object of the present invention, therefore, is the provision of meansforand a method of modulating the frequency of signals solely by electrical means having no moving parts,

Existing means for frequency modulation are unsatisfactory also because they produce undesired amplitude modulation. e 7

Another object of this invention, therefore, is the provision of means'for and a method of modulating frequency and eliminating amplitude variations. a

A further object of the invention is an improved The invention according to which the above 010-" jects are realized can best be understood from the following description considered in connection with the accompanying drawing, in which:

Figures 1 and 2 show diagrams which illustrate the principles on which this invention is based;

. frequency signal circuit is linked electrically and mechanically to the high frequency circuit, whereas'said circuits should be separate. 'For example, in a radio-telephone system, itis abmechanical impulses therebyrto activate the os- Mechani cal devices thus employed for'remote control method of varying the total effective resistance of an alternating current circuit containing pre- Figures 3 and 4 illustrate the embodiments of Y CONTROL New York, N. Y. 1931, Serial No. 567,120

(01. 179-111) f. V v V the invention in two oscillator circuits of different types; I .1

Figure 5 illustrates the embodiment of the invention in a. remote controlledfrequency modulated telephone circuit; I 3' r Figure 6 is the equivalent electricalcircuit of a control crystal and its mounting;

, Figure 7 is a radio-telegraphy circuit with crystal control and frequency modulation;

modulation in a telegraphtransmitter by movement of the crystal mounting-plate;

for frequency modulation by purely electrical means as well as for eliminating disturbing amplitude variations, isbased upon the following prinallel, as illustrated by. Figure l, qandb, the values of which, using complex symbols,are a: and y'y,"

respectively, the total circuitjmpedance Z of the network equals:

and the imaginary, or reactive component, '21, of Zis:*-'- '1: 2

k?+ y 7 If these impedancecomponents of the parallel circuit network.. are drawn as functions'of the variable resistance m, '(using, if desired, the fixed impedance y as unit of thesca1e), as shown in Fig. 2, e and j, in which the abscissae represent the variable resistance, r; th e ordinates, the resistive andthe reactive components of the'com bined-circuit impedance, Z'r and'zi, respectively, :we find that the resistance component first increases, reaches the maximum y/2, when :1: equals 1 and then decreases towards the value zero, which it reaches when :1! equals infinity. The corresponding reactance component steadily increases, passes thejrvalue 11/2 at its maximum rate of change, when w equals ygand ke'eps'on increasing towards it ultimate valued when x equals Figure 8, show an arrangementlfor frequency.

The present invention, which provides' m'ean's infinity. In the neighborhood of the values 1:11,

which means that a small change of :0 produces practically no change in the resistive component of circuit impedance. The differential of Z1 with respect to :r, ,inthesame neighborhood, :equals one-half, which means that a smallfchange of I .producesja relatively considerableichange in the reactive component of circuit impedance; this change being equal to one-half of the change of x.

Conversely, it can similarly be shown that if near the value 10:11, we vary the reactive member of the parallel connection slightly, the net effect on the total circuit impedance is not a change of the reactive, but of the resistive component. The

resistive component of the circuit impedance is as given by above Eq. 2: 7

x2+72. and in order to find the effect of a small change in y, we differentiate the above equation with respect to y, as set forth in the following equation:

For the case that y=rc we find:

, which shows a'definite effect upon the resistive impedance component. Now, in order to find the effect on the reactive component represented by above Eq. 3 as we differentiate this equation with regard to y, finding For the case that y =.'c, we find:

showing that the'difierential disappears, i. e., a small change of 1 has no effect upon the reactive impedance component.

A further inversion of the above principle is ,..obtained by connecting the resistive member and the reactive member in series, as shown in Figure 1, c and d. In this circuit connection, all statements above made for resistance in the parallel connection, are now valid for conductance; all

1 statements made for capacitive or inductive reactance are valid for admittance or susceptance. Thus, in parallel networks as illustrated, for example in Fig. 1, a and b, the network impedance:

In series circuits, illustrated in Fig. 1, c and d,

the impedance Zs=m+: and the network admitwhich is of exactly of the same form as the above equation, 7 I

From this formal identity it becomes clear that any relation derived between the impedances of parallel networks and the impedances of their and a variable modulating resistor function of the amplitude of oscillation. This amplitude adjusts itself so that the negative tube resistance is just large enough to compensate for the positive circuit damping resistance [6 and for the resistive component of the modulating network comprising the condenser l1 and the modulatlng resistor I8. On the basis of the preceding 'discussionit can be shown that if the resistance, r,

of the modulating resistor l8 approximately equals the reciprocal of the capacity of condenser l1 multiplied by the circular frequency of the oscillator, w, then a small change, d1", of the modulating resistance, r, produces a change in the efiective capacitive impedance component of network l1, N3 of the magnitude whereas the corresponding change in resistive impedance component equals which is small to the second order. Consequently, the amplitude variation produced by the change in modulator resistance is negligible, compared to the frequency variation caused by-the above change of modulator capacity. This is shown as follows: 7

The impedance (Z) of network I1, I 8:

where R=resistance of 18; and C=capacity of 17.

Y 3 F 1 R C W RCw y *1 +R C w 2RCW for

for

For'finite variations, resorting to the second diffor y,

ferential, since in accordance with Taylors series A 'dx' -cl x 1 2. Ax-AR /z(AR) dRp for d x 1 'dR ZR consequently,

d x an I 1 WA/3R2 dR 4R lhe above formula, impedance (Z) of network is derived as follows: According to" the above y +j y +y which may be rewritten as y'+jx I y x i-yi Substituting R for r and 2i, 2, plate battery 22, a filament battery2fiyan oscillator tuning coil 24, an oscillator tuning condenser 25, a feed-back coupling coil 26,2111 auxiliary or modulating inductance 2T anda varying modulating resistor 28. The action of the modulator comprising the inductance flandresistor 28 is the equivalent of the modulating condenser E? and modulating resistor l8 of the circuit shown in Figure 3, the difference being mainly that in the circuit, shown in Figure 4, inductance is used instead of capacity and a series connection of resistance and reactance isjemployed inst'eadof a parallel connection. j

As shown in Figure 5, the invention is applied to the remote control of a frequency modulated radio telephony circuit, and as described in connection with Figures 3 and 4, the modulation is accomplished by resistance change. In order to produce this resistance change by remote control and solely by electrical means, there is here utilized the principle that the plate resistance of a thermionic tube varies with the. grid voltage. As here shown, the circuit comprises a microphone 3!, a battery 32, an audio frequency cir "platefoscillator circuit, can be illustrated by'an equivalent electrical circuit. according to Figure 6.

cuit transformer 33, a modulating tube 34, a radio frequency choke coil 35, plate'battery' 36, a may ment batteryBl, a radio frequency coupling contuning coil 45, acompensating resistance 46, a

compensatingcoil Al and a compensating con- .denser'AB'. The micro-phone 3| and the battery 32'f'may be jremote'from the radio frequency circult and may be connected to the primary of the j transformer 33 by a' line-of arbitrary length; In operation, the'curren't variations produced by the [micro-phone 3! are transmitted to the primary of the" transformer 33 and through the medium of f .the secondary of the transformer corresponding voltage impulses are; impressed on the grid of modulator tube 34, producing corresponding ,J vchain'ge'sin its plate resistance. The choke coil vH lk has a very high impedance, compared tothe ,platejresistance, and prevents the plate battery .36 from forming a low, resistance by-pass for The modulator.

radio '7 frequency oscillations. tube 34 is coupled to the radio frequency circuit byv the small condenser 38 which has an impedarise at the oscillator frequency approximately equal to the plateresistance of tube 34. As previ- -o ously explained, the modulator produces on the circuitfa, resistive effect, which, although small, is proportional to the square of the modulator resistance change. In high qualitycircuits it is desirable to compensate for, even this small resis- H tive eifect;

' prising the compensating resistance 46, connect- For this purposethe network comedi inparallel with the compensating inductance iTandthe compensating condenser 48, which are series res'onant'at the operating mean frequency,

is provided, and bymeans thereof there is producediiithe circuit'an additional resistance adjus'table'in amount and proportion to the square of the frequency change thereby to compensate for the residual'resis'tive effect.

equal reciprocities, discussed in connection with The principle of Figure 1,0 and d can also be applied to the compensating network and accordingly a resistor connected in series to an antiresonant circuit, composed of an inductance and a capacitance in parallel, would havea conductive component equal to'the resistive component of the network shown indiigure 5,'.The adjustment of the network shown in Figure 5 is readily accomplished by first adjusting the variable resistor 46 to a medium value without signal impulses, tuning inductance Aland capacitance 48 to resonance with the Accordinggtothis invention'the methods herein disclosed can be'applied to frequency modulations of telegraph signals 'as'well as of telephone si nals." Thismay be illus'trated by oscillators utilizing crystal control, which in manyrespects are equivalent to free oscillators. A crystalitself without itsmounting is electricallyequivalent to a capacity shunted by a sharply series resonant circuit consisting of a very small denser 38," a radio frequency grid tuning conde'nser39, a radio frequency grid tuning coil 40, o

a radio frequency oscillator tube 4|, a plate batter'y'42, 'a filament'batteryfl, a radio frequency .plat'e 'tuning condenser 44, radio frequency plate capacity,,a very high inductance and a relatively small damping resistance.

The mounting, and

external circuit connectionsintr'oduce both series In this figure 5! is the equivalent of the mechanical resistance of the crystal, 52 is the equivalent of the mechanical inductance of the crystal, 53 is the equivalent of the mechanical capacitance of the crystal, 53a is the electrical capacitance of the crystal, 54 is the series electrical capacitance, 55 is the parallel electrical capacitance and 56 is the negative feed-back resistance introduced by the tube. It isknown that the frequency of crystal oscillators can be affected by varying the distance between the crystal and its mounting plate andfrequency modulating circuits based on this principle have been proposed. However,

while these circuits modulate the frequency they also affect the amplitude of modulation, in accordance with the above explained principle that in every circuit containing reactances and resistances a change of one of the reactancesalways produces a resistive effect.

In Figure '7, there is shown a circuit arrangement for frequency modulated radio telegraphy with a crystal controlled oscillator. As here shown the arrangement comprises an oscillator tube 6|, a plate battery 62, a filament battery 63, a grid bias battery 64, a plate tuning coil 65, a plate tuning condenser 66, a radio frequency choke coil 6! constituting a direct current by pass in the grid circuit, crystal mounting plates 68 and 69, an oscillating crystal 10, a grounding condenser l I, a modulating condenser I2, 2. modulating resistor 13 and a telegraph key or relay 14. This circuit with the exception of the grounding and modulating condensers, the modulating resistor, and the telegraph key is a standard crystal oscillator circuit well known to the art and requires no additional explanation.- The series ground capacity H is introduced in order to provide a circuit reactance external to the crystal impedance, which may be influenced by other external means, namely the series resistance and capacity 13 and 12 respectively. The telegraph key may be connected'across resistor 13, across the capacity 12 or across parts of either or both of them. In any case, the relative impedances of condenser 12 and resistor 13, with respect to each other and to the other circuit impedances, can be so adjusted that by short-circuiting part of condenser 12 or resistor 13, or parts, of both of them, only the tuning reactance of the equivalent resonant circuit is aifected and not the resistance.

In Figure 8 there is shown an arrangement for modulating a telegraph transmitter by moving the crystal mounting plate in a manner similar to that known in the art but with added means provided for eliminating the undesired amplitude modulation. As here shown, the arrangement comprises an oscillator tube 8 I, a plate battery 82, a filament battery 83, grid bias battery 84, plate tuning coil 85, plate tuning condenser 86, radio frequency choke coil 81, constituting a direct current bypass in the grid circuit, crystal mounting plates 88 and 89, oscillating crystal 90, and an adjustable capacitive plate 9| secured to the mounting plate 89 by the nuts 92 and 93. This oscillator is modulated by moving the upper mounting plate 88 upwardly or downwardly by.

ure 6 a decrease in capacity 54 is the equivalent of this movement. This movement of plate 88 also decreases the capacity between said plate and the auxiliary plate 9|, which is equivalent in Figure 6 to a reduction of capacity 55. This will also increase the oscillator frequency but will reduce the equivalent circuit resistance. By adjusting the original distance between plates 88 and 9|, this reduction of resistance may be made to compensate for the increase caused by the gap between crystal 90 and the plate 9!, thus producing pure frequency modulation without amplitude change. This can be understood more clearly by reference to Fig. 6, described above. In Fig. 8, the capacity of the gap between plate 88 and crystal 90 is the equivalent of capacity 54 in Fig. 6, and the capacity between plates 88 and 9| corresponds to a part of capacity 55 in Fig. 6. Now, it is a fundamental principle of oscillator circuits that their amplitude is stable only when the positive resistance of the working circuit is equal to the negative resistance supplied by the regenerating circuit. A preponderance of positive resistance will decrease the amplitude, a preponderance of negative resistance will increase the amplitude until the non-linearity of tube characteristic, etc., reestablishes the balance at a diiferent level. Now, if in a circuit of the type shown in Fig; 8 we attempt to vary the resonance frequency by a change in the series capacity 54 alone, this cannot be achieved without affecting the circuit resistance and thereby producing amplitude changes. Therefore, we apply the above stated principle a second time by simultaneously varying a second capacity, to wit, 55. A variation of 55 will affect the resistive component of the total net workjconsisting of parts 5| to 55. An increase of 54 will tend to increase the resistive component of net work impedance, whereas an increase ,of 55 will tend to decrease said component. If, by correct placing of 91 relative to 88, we make said decrease equal to said increase of resistive component, the result is a variation of the reactive component only of the combined network impedance; therefore, a variation of oscillating frequency only, without change in amplitude.

While I have shown and described several embodiments and applications of my invention it will be understood that the invention may be otherwise embodied and applied as will be understood by those skilled in the art. Therefore, I do not wish to be limited to the present disclosiu'e except as may be required by the appended claims and the prior art.

Having thus described the invention, What I claim is:

l. The method of varying the resistive component of the total effective circuit impedance of an alternating current circuit containing a component which is predominantly resistive and a component which is predominantly reactive which comprises varying the value of said predominantly reactive component in such a manner that the reactive component of the total effective circuit impedance remains substantially constant while the resistive component varies.

2. The method of varying the reactive component of the total effective circuit impedance of an alternating current circuit containing a component which is predominantly resistive and a component which is predominantly reactive which comprises varying the value of said predominantly resistive component, producing variations in said reactive component, in such a manner that the resistive component ofthe total j effective circuit impedance emainssub'stantially unaffected by said'variation.

3.'The method of modulating the frequency dominantly reactive which comprises varying the effective circuit reactance by varying the value of said Y while maintainingthe resistive compohentf of the total effective circuit impedance substantially constant. V v

4. The method of modulating the frequency of an electric wave in a radio frequency circuit having resistive and reactive components which comprises reactively coupling said circuit to the plate circuit of a thermionic tube and varying said reactive component by varying the plate resistance of said tube While maintaining the resistive component of the total effective circuit impedance substantially constant.

5. The method of modulating the frequency of an electric wave in a radio frequency circuit having resistive and reactive components which comprises reactively coupling said circuit to the plate circuit of a thermionic tube, and varying said reactive component by varying the plate resistance of the tube by varying the voltage impressed on the grid of said tube while maintaining the resistive component of the total effective circuit impedance substantially constant.

6. The method of modulating the frequency of an electric wave in a radio frequency alternating current circuit which comprises reactively coupling said circuit to the plate circuit of a thermionic tube, varying the resistance of said plate circuit to thereby vary the reactance of said radio frequency circuit, and introducing into said circuit a resistive effect to neutralize the resistive eifect produce by varying the resistance of said plate circuit.

7. The method of modulating the frequency of an electric wave in an alternating current circuit containing reactive circuit elements which comprises producing frequency changes by varying certain of said reactive circuit elements thereby producing an incidental resistive effect, and neutralizing said incidental resistive effect by simultaneously varying other of said reactive circuit elements to produce a resistive effect approximately equal to and opposite to said incidental resistive eifect.

8. The method of modulating the frequency of an electric wave in a radio frequency circuit having resistive and reactive components which comprises reactively coupling said circuit to the plate circuit of a modulator tube, and varying said reactive component by varying the plate circuit resistance of said tube in accordance with signal controlled voltages impressed on the grid of said modulator tube while maintaining the resistive component of the total effective circuit impedance substantially constant.

9. In combination a modulator circuit including a thermionic modulator tube, a grid circuit, and a plate circuit; a radio frequency circuit including a thermionic oscillator tube, a grid circuit and a plate circuit, and a coupling between the plate circuit of said modulator tube and the grid circuit of said oscillator comprising a reactance the impedance of which at the oscillator frequency approximately equals the plate resistance of said modulator tube.

10. In combination a modulator circuit in- ;d ominantly resistive component 7 changes. in

eluding a thermionic modulator tube, a grid circuitfiand a plate vcircuit; a radio frequency circuit including atherimonicoscillatoritube, a grid circuit andaplate circuit, and a coupling between theplatejcircuit of said modulator tube, thegrid circuit of said oscillatorjcomprisinga reactance the impedance of which at the oscillator frequency approximately equals the plate'resistance of said modulator tube, and a resonant network connected to said radio frequency circuit for ne'u tralizi'ng the resistiveeffect. produced therein by the plate resistance ofthe modulator tube. p w

11. In combination a modulator circuit including a thermionic modulator tube, a grid circuit, and a, plate circuit; a radio frequency circuit including a thermionic oscillator tube, a grid circuit and a plate circuit, and a coupling between the plate circuit of said modulator tube and the grid circuit of said oscillator comprising a reactance the impedance of which at the oscillator frequency approximately equals the plate resistance of said modulator tube and means for preventing amplitude variations in said radio frequency circuit.

12. In combination a modulator circuit including a thermionic modulator tube, a grid circuit, and a plate circuit; a radio frequency circuit including a thermionic oscillator tube, a grid circuit and a plate circuit, and a coupling between the plate circuit of said modulator tube and the grid circuit of said oscillator comprising a reactance the impedance of which at the oscillator frequency approximately equals the plate resistance of said modulator tube, and means for introducing into said radio frequency circuit a resistive eifect to neutralize the resistive effect produced therein by changes in the plate resistance of said modulator tube.

13. An oscillator circuit, a thermionic tube having a plate circuit and a grid circuit, a reactance coupling between the oscillator circuit and said plate circuit, means for varying the resistance of said plate circuit, and means for preventing amplitude variations of the currents flowing in said oscillator circuit due to changes in said plate circuit resistance including means for compensating for said resistance changes.

14. A radio telephone circuit comprising a modulator tube circuit having grid and plate circuits, a micro-phone circuit inductively coupled to said grid circuit, a radio frequency oscillator circuit reactively coupled to said plate circuit, whereby changes in the plate circuit resistance of said modulator tube circuit produced in accordance which voltage variations on said grid cause reactance changes in said oscillator circuit, and means for preventing amplitude variations in said oscillator circuit comprising means for compensating for resistance variations in said oscillator circuit.

15. A crystal controlled oscillator having means for varying the frequency comprising a crystal, relatively movable mounting plates therefor, and an auxiliary plate adjustable to prevent changes in amplitude due to relative movement of said mounting plates.

16. In an alternating current circuit having a resistive component and a reactive component, the method which comprises varying the value of one of said components by varying the other component in such a manner that the component of the total effective circuit impedance similar to said other component remains substantially constant.

17. The method of modulating the frequency of the electric wave in an alternating current circuit containing a component which is predominantly resistive and a component which is predominantly reactive which comprises varying the efiective circuit reactance by varying the value of said predominantly resistive component while maintaining the resistive component of the total efiective circuit impedance substan tially constant.

18. In combination a modulator circuit including a, thermionic modulator tube, a grid circuit, and a plate circuit; a radio frequency circuit including a thermionic oscillator tube, a grid circuit and a plate circuit, and a coupling between the plate circuit of said modulator tube, the grid circuit of said oscillator comprising a reactance the impedance of which at the oscillator frequency approximately equals the plate resistance of said modulator tube, and a resonantnetwork including a variable resistance connected to said radio frequency circuit for neutralizing the resistive efiect produced therein by changes in the plate resistance of the modulator tube.

CARL FICHANDLER. 

